Split changelog into one file per release

[andygrove]

Is your feature request related to a problem or challenge?

The changelog file is large and is cumbersome to edit. GitHub currently displays an error This blob took too long to generate. when trying to render the changelog.

Describe the solution you'd like

• Create a new top-level changelog directory (it could also go under dev).
• Create one file per release e..g changelog/21.0.0.md
• Update the existing datafusion/CHANGELOG.md file to contain links to each release

Describe alternatives you've considered

No response

Additional context

No response